The present invention pertains to the field of methods and apparatuses relating to radio communication; and in particular to the part of this field that concerns cellular radio communication.
A conventional public land mobile telephone network (PLMN)xe2x80x94i.e. cellular radio communication systemxe2x80x94include a land system and one or more radio terminals, e.g. mobile (or fixed) cellular radio phones, cellular communicators or computers having associated cellular radio communication equipment. The land system typically includes a switching system, which is connected to one or more radio communication nodes, commonly referred to as radio base stations (RBS). The switching system includes one or more switching nodes, for example mobile services switching centres (MSC), which are arranged for providing the necessary switching functions in order for information (e.g. coded speech or data) to be transferred in a desired manner between the radio base stations. The switching system includes registers containing, among other things, information relating to subscriptions in the PLMN. The switching system normally includes also one or more gateway nodes for connecting the PLMN to other communication networks, e.g. public switched telephone networks (PSTN), data networks, Internet et cetera. Each radio base station serves at least one geographical area known as a cell. The radio base stations are arranged for providing radio communication services (e.g. speech communication or data communication) to radio terminals that are within the cell(s). The radio terminals communicate with the radio base stations via a predetermined radio interface. The land system is arranged for allowing radio terminals to communicate, via the radio base stations and the switching system, with, for example, other radio terminals in the PLMN, with telephones connected to a PSTN or with computers connected to a data network or internet.
In order to organise radio channels for the radio communications between the radio terminals and the radio base stations, a number of multiple access methods have been developed. Today the most widely used access methods are FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access).
In FDMA, an available radio frequency range is divided into a number of (narrow) separated frequency bands. During FDMA communications between a radio terminal and a radio base station, a selected first frequency band is used for uplink communication (radio terminal to radio base station) and a selected second frequency band is used for downlink communication (radio base station to radio terminal). The first frequency band and the second frequency band are dedicated for the communication between the radio terminal and the radio base station and are not at the same time used for communications between the radio base station and other radio terminals in the cell(s) associated with the radio base station. FDMA is used foremost in older analogue PLMN systems, for example in the NMT system (Nordic Mobile Telephone system) and in the AMPS system (Advanced Mobile Phone System).
In TDMA, an available frequency range is normally also divided into a number of separated frequency bands. During TDMA communications between a radio terminal and a radio base station, a selected first frequency band is used for uplink communication and a selected second frequency band is used for downlink communication. However, the radio terminal is not allowed to use the first frequency band for uplink communication all the time, but only during predetermined time intervalsxe2x80x94so called time slots. In a similar way, the radio base station is only allowed to use certain time slots for the downlink communication with the radio terminal. The remaining time slots of the first frequency band and the second frequency band are available for uplink and downlink communication between the radio base station and other radio terminals in the cell(s) associated with the radio base station. TDMA is used in several digital PLMN, for example the GSM system (Global System for Mobile communications) and the D-AMPS system (Digital-AMPS (TDMA)).
The CDMA methods are somewhat more complicated than FDMA and TDMA. However, a common feature of the CDMA methods is that transmitted radio signals have a frequency bandwidth which is large in comparison with the bandwidth of the information that is transferred by the radio signals (band spread signals). The most common CDMA methods are frequency hopping CDMA and DS (Direct Sequence) CDMA. For example, the TIA (Telecommunications Industry Association) interim standard IS-95 makes use of DS access.
In the PLMN, ongoing communications between the radio terminals and the radio base stations must not be unintentionally lost, and the communication quality must be acceptable to users of the radio terminals. However, there are several known transmission problems that may cause communications to be lost and communication quality to be degraded. Some of the main transmission problems are path loss (i.e. a decreased signal strength due to an increased distance between the radio terminal and the radio base station), interference from other radio transmissions and Rayleigh fading (i.e. decreased signal strength due to destructive interference caused by multipath propagation of radio signals).
The normal remedy for path loss is to perform an inter-cell handoff, i.e. a change of channel from the channel associated with the radio base station via which the radio terminal is currently communicating to a second channel associated with a new radio base station which is closer to the radio terminal.
When interference is present, the received signal strength (on up- and/or downlink) is normally not low but the communication quality is nevertheless poor, e.g. the bit error rate (BER) and/or frame erasure rate (FER) are high. Inter-cell handoff is therefore not a suitable remedy, since low signal strength is not the problem. The normal remedy is instead a change of channel from the currently used channel to new channel associated with the same radio base station but using different frequencies than the currently used channel. Such a change of channel is commonly referred to as an intra-cell handoff.
The PLMNs has been used foremost with moving radio terminals, such as mobile cellular radio phones. Rayleigh fading has therefore not been considered a severe transmission problem, since signal loss due to Rayleigh fading only occurs for fairly short periods of time, which normally does not jeopardise the communication quality to any great extent.
The present invention addresses mainly the problem of improving reliability and communication quality in a cellular radio communication system.
According to the present invention it is observed that today it is becoming more common to have stationary or slowly moving radio terminals. For example, the use of computers having associated cellular radio communication equipment and stationary cellular phones (e.g. in communication units in dispense or slot machines and the like) is becoming more frequent. For stationary and slow moving radio terminals Rayleigh fading is more likely to cause the loss of communications or severely degraded communication quality, especially in FDMA and TDMA systems. Rayleigh fading influences CDMA systems negatively to a lesser degree than FDMA and TDMA systems. However, in indoor environments the coherence bandwidth is normally large, and Rayleigh fading may therefore influence also band spread signals negatively to a non-negligible extent.
The problem stated above is therefore solved in short by taking into account the negative influences of Rayleigh fading and actively responding in an appropriate manner to such influences.
A main object of the invention is thus to improve reliability and communication quality in cellular radio communication systems, and the invention includes methods as well as apparatuses for achieving this object.
The problem stated above is solved in somewhat more detail according to the following. It is determined whether one of an uplink or a downlink of a radio channel is subject to a Rayleigh fading dip. If it is determined that one of the uplink or the downlink is subject to a Rayleigh fading dip, it is then determined whether it is necessary to execute a countermeasure in order to avoid the negative influences of Rayleigh fading on the channel. The invention is naturally not limited to improving conditions on one channel but may be employed to any number of channels used for communications in a PLMN.
In addition to solving the above-stated problem, the invention has the advantage that it provides a more optimal usage of available channel resources, thereby increasing the coverage of the cellular radio communication system.
The invention will now be described further using preferred embodiments and referring to the drawings.